An aerobic biological treatment of aqueous organic wastes by treating them under an aerobic condition by making use of the actions of aerobic microorganisms, as in a biosludge treatment, permits a low cost treatment with a superior treatment performance and has found a wide use in general. It suffers, however, from a problem of the occurrence of a large amount of "excess sludge", which is difficult to dewater. This excess sludge may amount to about 30-60% by weight of the reduced BOD and the disposal thereof is difficult. Heretofore, the excess sludge has been disposed of by depositing it in a landfill. It has recently become more and more difficult to reserve such a landfill site and, thus, a reduction of the amount of excess sludge is required.
A process has been proposed for reducing the amount of excess sludge in an aerobic biological treatment of an aqueous organic waste by subjecting the biosludge formed therein to an ozone treatment and returning the resulting treated sludge to the aerobic biological treatment step (for example, Japanese Patent Kokai No. 206088/1994 and corresponding EPO 0645347 A1). This prior technique can reduce the amount of the biosludge formed during the aerobic biological treatment by subjecting the biosludge to an ozone treatment to convert it into an easily biodegradable product to be metabolized by aerobic microorganisms as a BOD source, whereby it may eventually be attainable to reduce the amount of formed excess sludge to zero.
In such a treating technique, the ozone treatment is realized by blowing an ozone-containing gas into a biosludge-containing liquor to cause the ozone to contact with the biosludge. When an ozone-containing gas is blown into a biosludge-containing liquor, the liquor becomes viscous and easily foamable due to the oxidative degradation of the biosludge, whereby the entire biosludge-containing liquor in the ozone-treating vessel will become filled with a foam layer of a consistency similar to shaving cream. Even if the ozone-containing gas is further blown thereinto in this state, it is difficult to increase the gas/liquid contacting efficiency by a further fine disintegration of bubbles and, thus, any higher ozone absorption rate may not be attained.
Therefore, it is necessary, for attaining a high ozone absorption rate, to further disintegrate the bubbles of the ozone-containing gas, wherefor a technique of blowing an ozone-containing gas into the biosludge-containing liquor through a porous gas diffuser plate may be employed. Here, however, bacteria having a high resistivity against ozone are apt to grow due to the conversion of the biosludge into an easily biodegradable product, despite the ozone exhibiting a bactericidal action. This will bring about the adhesion of a biofilm onto the diffuser plate to cause the blockage thereof, so that the introduction of the ozone-containing gas therethrough may not be allowed to be maintained for a long period of time.
An object of the present invention is to provide a process and an apparatus for a biological treatment of aqueous organic wastes, which enables the ozone absorption rate to be maintained at a higher value and to realize an uninterrupted long-lasting introduction of the ozone-containing gas by introducing the ozone-containing gas so as to form finely disintegrated bubbles without suffering from blockage of the gas blow-in element, even when a thick foam layer is built up within the entire ozone-treating vessel due to the formation of a viscous and easily foamable biosludge suspension by the ozone treatment, whereby an efficient ozone treatment can be realized while reducing the amount of excess sludge.